1. Field of the Invention
The invention relates to a Michelson interferometer comprising a beam splitter, two stationary plane mirrors enclosing an angle of 90.degree. with each other and of 45.degree. with the beam splitter in each case, a collecting lens, a detector for the signal radiation, a laser reference unit consisting of a laser and a laser detector and two retroreflectors or two roof edge mirrors. The invention also concerns a Michelson interferometer comprising two plane mirrors and a rotating retroreflector with associated drive motor, the retroreflector rotation axis being offset laterally with respect to the triple point of the retroreflector, two deflection mirrors, a beam splitter, a collecting lens, a detector and a laser reference unit comprising laser and laser detector. Such an interferometer is used in Fourier transform spectroscopy (FTS) in which optical path differences are generated by rotating retroreflectors.
2. Description of the Prior Art
Michelson interferometers are known which are also referred to as "Fourier spectrometers" in which optical path differences are generated by rotating retroreflectors (for example DE 40 05 491 A1). Eccentrically and inclined "nutating" retroreflectors are used. To generate relatively large optical path differences, i.e. a higher spectral resolution, two or more retroreflectors are set in asynchronous rotation with respect to the optical path change in the two interferometer arms, and a fixed phase relationship of the different rotational movements to each other must be maintained.
The known Michelson interferometers with rotating retroreflectors have however the following disadvantages:
1. To obtain a high spectral resolution the interferometer cannot be operated with only one reflector; two or more of these relatively expensive elements are required. PA1 2. Complicated steps must be taken for the necessary maintenance of the fixed phase relationship of the different rotational movements of the two or more retroreflectors with respect to each other. A very advantageous solution is the drive of each retroreflector with its own stepping motor and the "electrical coupling" of all the motors to each other by means of only one common control clock for all motors. However, this in turn has the following disadvantages: The mechanical vibrations of the stepping operation of the motors is transmitted to the entire structure in a manner disturbing for the measuring signal; these vibrations must therefore be eliminated by additional complicated mechanical damping steps. Furthermore, the total expenditure is increased by each additional drive motor. Also, stepping motors are fundamentally more expensive than comparable DC voltage motors which suffice for the operation with only one retroreflector. PA1 3. Each further retroreflector has optical errors which lead to an additional impairment of the quality of the optical signal of the apparatus. This could be avoided only by retroreflectors of extremely high quality; this would result however in an extremely expensive apparatus. PA1 4. To avoid undesirably large mechanical dimensions of the apparatus with the known interferometers having only one rotating retroreflector, in the second interferometer arm as well a (however) fixed retroreflector is installed by which in particular the optical path is convoluted and a small mechanical overall size thereby made possible. However, this results in all the disadvantages already described above due to the use of a second retroreflector.
If two or more retroreflectors are driven by only one motor, coupling via a transmission is necessary. This again makes the expenditure greater and introduces additional interference sources into the construction due to transmission play, which cannot be completely excluded.
The invention therefore has as its object the provision of a Michelson interferometer which with relatively low expenditure permits a high resolution.
The invention therefore proposes in a Michelson interferometer comprising a beam splitter, two stationary plane mirrors enclosing an angle of 90.degree. with each other and 45.degree. with the beam splitter in each case, a collecting lens, a detector for the signal radiation, a laser reference unit consisting of a laser and a laser detector and two retroreflectors, the improvement in which in each arm of the interferometer a retroreflector having its aperture plane aligned perpendicularly to the optical axis is mounted on a respective holder which in turn is rigidly connected to one end of a shaft rotatably mounted in a connecting member and to the other end of which a first gear is secured which is coupled via a toothed belt to an identically configured second gear which lies in the plane of the first gear and which is arranged concentrically to a drive shaft of a stationarily arranged electric motor and is rigidly connected to the housing thereof, the respective drive shaft being fixedly connected at a predetermined distance from the rotatably mounted shaft to the connecting member in such a manner that on rotation of the two drive shafts of the two electric motors the optical path length in one interferometer arm is shortened and in the other interferometer arm is lengthened synchronously therewith and the aperture planes of the two retroreflectors always remain unchanged aligned perpendicularly to the optical axis.
The invention also proposes in a Michelson interferometer comprising a beam splitter, two stationary plane mirrors enclosing an angle of 90.degree. with each other and 45.degree. with the beam splitter in each case, a collecting lens, a detector for the signal radiation, a laser reference unit consisting of a laser and a laser detector and two 90.degree. roof edge mirrors, the improvement in which in each arm of the interferometer a 90.degree. roof edge mirror having its aperture plane aligned perpendicularly to the optical axis is mounted on a respective holder which in turn is rigidly connected to one end of a shaft rotatably mounted in a connecting member and to the other end of which a first gear is secured which is coupled via a toothed belt to an identically configured second gear which lies in the plane of the first gear and which is arranged concentrically to a drive shaft of a stationarily arranged electric motor and rigidly connected to the housing thereof, the respective drive shaft being fixedly connected at a predetermined distance from the rotatably mounted shaft to the connecting member in such a manner that on rotation of the two drive shafts of the two electric motors the optical path length in one interferometer arm is shortened and in the other interferometer arm is lengthened synchronously therewith and the aperture planes of the two 90.degree. roof edge mirrors always remain unchanged aligned perpendicularly to the optical axis, and in which the aperture of each 90.degree. roof edge mirror is twice the diameter of the beam passing through the interferometer and length (along the roof edge) is equal to the diameter of a rotation circle of the 90.degree. roof edge mirror plus the diameter of the beam.
The invention also proposes in a Michelson interferometer comprising a beam divider, two stationary plane mirrors enclosing an angle of 90.degree. with each other and 45.degree. with the beam splitter in each case, a collecting lens, a detector for the signal radiation, a laser reference unit consisting of a laser and a laser detector and two retroreflectors, the improvement in which in each arm of the interferometer a retroreflector having its aperture plane aligned perpendicularly to the optical axis is mounted on a respective holder which in turn is secured to a gear which is mounted rotatably on a shaft-like extension of a connecting member on different sides of the connecting member two identically configured gear pairs each comprising two different gears coupled by a respective toothed belt are arranged, the first gears of each gear pair being secured to the two ends of a shaft mounted in the connecting member whilst substantially opposite the gear rotatably mounted on the extension of the connecting member the second gear of a gear pair facing a stationarily arranged drive motor is secured concentrically to the drive shaft thereof to the motor housing, the respective drive shaft being fixedly connected at a predetermined distance from the rotatably mounted shaft to the connecting member in such a manner that on rotation of the two drive shafts of the two electric motors the optical wavelength in one interferometer arm is shortened and in the other interferometer arm is lengthened synchronously therewith and the aperture planes of the two retroreflectors always remain unchanged aligned perpendicularly to the optical axis.
The invention also proposes in a Michelson interferometer comprising two plane mirrors, a rotating retroreflector with associated drive motor, the retroreflector rotation axis being laterally offset with respect to the triple point of the retroreflector, two deflecting mirrors, a beam splitter, a collecting lens, a detector and a laser reference unit with laser and laser detector, the improvement in which the rotating retroreflector is arranged as sole retroreflector for two interferometer arms in such a manner that both beam halves split by the beam splitter and incident on a respective deflecting mirror are deflected into aperture subregions of the single retroreflector which lie opposite each other with respect to the retroreflector rotation axis, the optical axes of the two beam halves being inclined to each other an angle of 2.alpha. and to the retroreflector rotation axis an inclination angle .alpha..
Advantageous further developments of the invention are the subject of the subsidiary claims.
According to the invention an interferometer structure is provided in which a high spectral resolution is implemented with low technical expenditure. Due to the relatively simple construction balancing can easily be carried out. Furthermore, the beam in operation is incident only on mirror surfaces of the same inclination so that no changing polarizations can occur.
A further preferred embodiment of an interferometer contains only one rotating reflector. The high spectral resolution achieved with low technical expenditure corresponds to that which in other constructional forms could be achieved only with two rotating retroreflectors. In spite of the low expenditure, at the same time the possible interference sources are reduced. Since both beam halves run through the same reflector they are "optically coupled". The drive can therefore be effected with a simple direct current motor without a transmission causing disturbances or any stepping controls also causing disturbances, and is thus also technically less complicated.